1. Field of the Invention
The invention relates to an improved actuator unit comprised of a piezoelectric actuator.
2. Description of the Prior Art
Actuator units of the type with which this invention is concerned are use among other things in fuel injection systems and in particular in fuel injection valves since the switching times of such actuator units are very short. The short switching times permit a more exact metering of the injected fuel quantity and permit an improved shape of the injection curve over time. In connection with the present invention, the generic term “fuel injection valve” is understood to mean all types of fuel injection values, for example, the injectors for common rail injection systems or injection nozzles of conventional fuel injection systems. A fuel injection valve with a piezoelectric actuator is triggered by applying an electric voltage to the piezoelectric actuator; this causes a rapid expansion of the piezoelectric actuator due to known physical effects of piezoelectric ceramic and causes a valve-closure member to lift away from a valve seat. The piezoelectric actuator has a certain mass that is accelerated in the course of this. If the voltage applied to the actuator is reduced, this causes the actuator to contract. As a function of the triggering speed, the inertia of the previously accelerated mass of the actuator generates damaging tensile forces in the piezoelectric actuator, in particular causing fractures in the soldered connections between the individual layers of the piezoelectric actuator. In order to prevent this kind of damage, it has become common practice to use a cylindrical hollow body in the form of a spring to prestress the piezoelectric actuator in the axial direction. A device of this kind is known, for example, from WO 00/08353 (Siemens). This hollow body is bent from a flat metal sheet and welded at the first joint thus produced. The first joint extends parallel to the longitudinal axis of the hollow body.
Among other things, the welding of the first joint has the following disadvantages: the welding causes a generally undesirable structural change to the hollow body in the immediate vicinity of the welding seam. A second problem is the spatters produced during the welding, which can lead to difficulties in assembly of the actuator unit or can even lead to functional failures of the fuel injection valve when one or more spatters come loose during operation. A third problem is a sinking-in of the welding seam (seam sinkage) at the beginning and end of the welding seam and the resulting notch effect and voltage spikes.